Real-life biological, environmental or chemical samples frequently contain a large number of molecules of differing molecular sizes and weights. A few examples of such samples are bodily fluids such as blood, urine and saliva or the contents of a cell. The size of these particles can range from 0.1 mm to less than 1 nm. The presence of particles spanning such a wide range can create a number of problems in miniaturized systems such as blockage of fluidic channels and adsorption of unwanted molecules on system surfaces (channel fouling). Furthermore, in typical applications, it is often desirable to analyze specific classes of molecules (e.g., proteins); eliminating other particles (e.g., cells, and cell fragments) in order to reduce the background “clutter” in the sample and thereby simplifying analysis and providing greater sensitivity. In particular, in biomedical applications in order to study cell proteins and signaling molecules, the cell membrane must be ruptured and the contents of the cell released. In practice, cell samples are typically opened by mechanical emulsion or by exposing the cell sample to a denaturing solution. In doing so one is left with a myriad of particles and molecules that must be filtered in order to be analyzed.
Dialysis is a means of separating molecules using a porous membrane. The separation is achieved according to molecular size or molecular weight of the assemblage of molecules under study: molecules smaller than the membrane pore size will pass through the membrane, while larger molecules are excluded. Dialysis, therefore, can be applied to achieve either of two purposes: (a) to remove interfering compounds, contaminants, or salts from a biological sample; or (b) to extract those molecules of interest from a “dirty” sample or a crowded assemblage of materials. In the former case, the molecules that do not pass through the membrane are of interest while in the latter case those molecules that do move through the membrane are of interest. The driving force for dialysis is the concentration differential between the solutions (sample and perfusion liquid respectively) on either side of the membrane. (For filtration, the process is the same but the driving force is a pressure gradient.) For maximum efficiency, the membrane is made to be as thin as possible while still providing sufficient rigidity and strength to prevent membrane rupture. Moreover, the concentration differential across the membrane is maintained as large as possible, and the membrane pore size distribution is made as narrow as possible such that the “tails” of the distribution decline rapidly.
Microfluidic devices (specifically, those constructed using glass wet-etching, silicon micromachining, or LIGA-type processes) have in many ways revolutionized the analytical and synthetic capabilities available for chemistry, biology, and medicine (the term “microfluidics” is herein intended to imply fluidic processes occurring in fluid channels having cross-sectional dimensions below 1 mm and lengths ranging from millimeters to tens of centimeters). A number of analytical techniques have been shown to perform better in microfluidic structures of this type, and synthesis of small structures using the minimum amount of reagents requires efficient use of materials in small channels. Microfluidic devices allow analysis using minute amounts of samples (crucial when analyzing bodily fluids or expensive drug formulations), are fast and enable development of portable systems.
When dealing with small volume samples, however, one of the major problems is a loss of sample due to the transfer of samples to and from the dialysis equipment. When sample is present in such a small volume and not readily available the loss of sample becomes an important consideration.